Construct values lazily

[alastairreid]

Lazy construction will work better with concolic execution because it defers path splitting to the point where a value is accessed.

This cannot be done for types like Option
But it can possibly be done for abstract Types like BTreeMap - possibly depending mutability?

[alastairreid]

Two directions to take this:

1. Rust's lazy statics potentially have the opposite problem: deferring construction of an object until after a path split - which requires every path to repeat the effort of creating the object for itself. Perhaps the initialize method would help with this?

2. Chopped symbolic execution adds lazy evaluation into KLEE and might be worth investigating.